In modern aircraft, the cabin is typically lined with thin honeycomb panels that are attached to underlying structure. The attachments are accomplished by means of hot melt fasteners that are adhesively bonded to the back of the panels and have projecting studs that engage the structure. A typical hot melt fastener has a thin round base plate with a diameter of about 5/8 to one inch and a threaded stud projecting therefrom. A thermal or hot melt adhesive layer is bonded to the base plate on the back side opposite the stud. Hot melt induction heaters are used to heat the base plate and thereby melt the adhesive and bond the fastener to a sandwich panel.
Induction heaters for heating fastener base plates have been used for a number of years. Such a heater is disclosed in U.S. Pat. No. 4,355,222, granted Oct. 19, 1982, to Glenn A. Geithman et al. An earlier induction heater design is disclosed in U.S. Pat. No. 3,845,268, granted Oct. 29, 1974, to Melvin R. Sindt. The latter was designed for use with a different type of hot melt fastener which has, instead of the projecting threaded stud, a flange on the adhesive side that is received into an opening in the panel.
FIG. 4 is a simplified circuit diagram of the control system for the type of heater disclosed in U.S. Pat. No. 4,355,222. Referring to FIG. 4, a connector 2 (e.g. a plug) is connected through a fuse 4 to a main power switch 6. The system is activated by the operator pushing a trigger 12 that initiates an electromechanical timer 10 connected to the main switch 6 through a relay 8. Isolation transformer 14 is also energized by operation of the trigger 12. The transformer 14 provides a source of power that is isolated from the line for safety reasons. The transformer 14 is connected to a bridge 16 which, in turn, is connected to a power oscillator circuit 18, including a capacitor C1, coil 26 and coil 27. The power circuit 18 provides power to a work coil 26 which, when the heater is in use, supplies electromagnetic energy to the fastener base plate to heat the plate. A silicon controlled rectifier (SCR) 20 controls the energy pulses to the work coil 26. The system converts 110 volt alternating current to direct current and then creates a 35 kilohertz (KHz) signal.
A preset pulse train received by the SCR 20 determines the duty cycle of the electromagnetic pulses to the work coil 26, i.e. the on and off times of the pulses. In the system shown in FIG. 4, the duty cycle is about 5 KHz. The shorter the time between the 35 KHz signals, the higher the output power to the work coil 26. At the end of the heating time (about eight seconds), the timer 10 signals the end of the heating time by turning the unit off. During the operation of the heater, a control circuit 22 provides the pulse train to SCR 20. When the unit is turned off, stored energy is gradually discharged by elements 24, including capacitor C2 and resistor R2. The additional components and operation of the known system shown in FIG. 4 is further described in U.S. Pat. No. 4,355,222.
The system shown in FIG. 4 is intended to heat fasteners to approximately 460.degree. F. This desired temperature provides the proper bonding of the fastener to the panel. A problem that has been encountered in connection with the known heater is that the units that are actually used tend to drift from their optimum setting, resulting in either overheating or underheating of the fasteners. Overheating creates shiny spots on the side of the panel facing the cabin which are blemishes on the decorative panels. Underheating results in low bond strength. Temperature variations are caused by a number of process variables. Such variables include line voltage variations, differences in the types of panels to which the fasteners are bonded, differences in the induction heating properties of the fasteners, variation in the efficiency of the electronic heater components over time, and operator hand pressure. Because of the lack of feedback in the system represented in FIG. 4, the known unit cannot compensate for most of these process variables.